2-D electronics: researchers improve hexagonal material

LONDON – The possibility of creating superior electronic devices that operate in single-atom thick layers of semiconductor material has been advanced by work conducted by scientists from Rice University and Oak Ridge National Laboratory (ORNL). The team has found it can improve the creation of monolayers of molybdenum disulphide in a chemical vapor deposition (CVD) furnace by deliberately introducing imperfections to the substrate.

When that progress is added to previous work on graphene and hexagonally-ordered boron nitride it enhances the prospect of being able to produce tailored crystalline solids that could be optimized for a range of devices including field-effect transistors, integrated logic circuits, photodetectors and flexible optoelectronics, the team said.

The latest work by the researchers was reported in Nature Materials.

The performance of a 2-D electron gas at the interface between two dissimilar materials has been studied for many years and graphene, where carbon is arranged in hexagonally in a single-atom layer, has shows exceptional electron mobility, thermal conductivity and strength. However, graphene, a pre-eminent conductor, is not the only material whose atoms can be arranged in a hexagonally-ordered layer. Molybdenum disulfide (MDS) is a semiconductor and hexagonal boron nitride (hBN) is an insulator.

Research is now looking at the combination of these three materials as a means of making functioning 2-D electronic devices. While members the research team had previously reported on the combination of graphene and hBN, MDS has proved difficult to grow. Early CVD experiments produced crystal grains that were too small to be useful, the team said.

The latest work, was conducted by Jun Lou, Pulickel Ajayan and Boris Yakobson, all professors in the university’s Mechanical Engineering and Materials Science Department, who collaborated with Wigner Fellow Wu Zhou and staff scientist Juan-Carlos Idrobo at ORNL.

"The material is difficult to nucleate, unlike hBN or graphene," said co-author Sina Najmaei, a Rice graduate student, in a statement. "We started learning that we could control that nucleation by adding artificial edges to the substrate, and now it's growing a lot better between these structures."

"Now we can grow grain sizes as large as 100 microns," Lou said, in the same statement. "That's still only about the width of a human hair, but in the nanoscale realm, it's big enough to work with," he said.

Schematics (left) and experimental images (right) produced by
Oak Ridge National Laboratory show defects in
two-dimensional samples of molybdenum disulfide. Source: Rice University.

Once the Rice teams were able to grow such large MDS grains the ORNL
team imaged the atomic structures using aberration-corrected scanning
transmission electron microscopy. This was then used to check the energy
level of regular and defect structures as well as confirm previously
predicted phenomena such as the existence of conductive "wires"
operating along grain boundaries in MDS.

With germanane, graphene, conductive plastics, and molybdenum disulfide, this is going to be the decade of new electronic circuit materials. Besides flexible electronics, better manufacturing yields, and speed, I wonder what the other consequences will be.